The spectral properties from radio to optical bands are compared between the 18 optically bright Gamma-ray burst afterglows and well established powerspectrum sequence in Blazars. The comparison shows that the afterglows are well agreement with the well known Blazar sequence (i.e., the νL ν (5GHz)-α RO correlation, where α RO is the broad-band spectral slope from radio to optical bands). The afterglows are, however, clustered at the low luminosity end of the sequence, which is typically occupied by high frequency-peaked BL Lac objects. The correlation suggests that Gamma-ray burst afterglows share the similar emission process with high frequency-peaked BL Lac objects. We further identify a deviation at a significance level larger than 2σ from the sequence for three typical optically "dark" bursts. The deviation favors a heavy extinction in optical bands for the "dark" bursts. The extinction A V is estimated to be larger than 0.5-0.6 magnitude from the νL ν (5GHz)-α RO sequence.
We study the AGN-host co-evolution issue here by focusing on the correlation between the hard X-ray emission from central AGNs and the stellar populations of the host galaxies. By focusing on the galaxies with strong Hα line emission (EW(Hα) > 5Å), both X-ray and optical spectral analysis are performed on 67 (partially) obscured AGNs that are selected from the XMM-Newton 2XMMi/SDSS-DR7 catalog originally cross-matched by Pineau et al. The sample allows us to study central AGN activity and host galaxy directly and simultaneously in individual objects. Combining the spectral analysis in both bands reveals that the older the stellar population of the host galaxy, the harder the Xray emission will be, which was missed in our previous study where the ROSAT hardness ratios are used. By excluding the contamination from the host galaxies and from the jet beaming emission, the correlation indicates that the Compton cooling in the accretion disk corona decreases with the mean age of the stellar population. We argue that the correlation is related to the correlation of L/L Edd with the host stellar population. In addition, the [O I]/Hα and [S II]/Hα narrowline ratios are identified to correlate with the spectral slope in hard X-ray, which can be inferred from the currently proposed evolution of the X-ray emission because of the confirmed tight correlations between the two line ratios and stellar population age.
The origin of excess of X-ray column density with respect to optical extinction in gamma-ray bursts (GRBs) is still a puzzle. A proposed explanation of the excess is the photoelectric absorption due to the intervening clouds along a GRB's line of sight. Here, we test this scenario by using the intervening Mg ii absorption as a tracer of the neutral hydrogen column density of the intervening clouds. We identify a connection between the large X-ray column density (and large column density ratio of log(N H,X /N H i ) ∼ 0.5) and large neutral hydrogen column density probed by the Mg ii doublet ratio (DR). In addition, GRBs with large X-ray column density (and large ratio of log(N H,X /N H i ) > 0) tend to have multiple saturated intervening absorbers with DR < 1.2. These results therefore indicate an additional contribution from the intervening system to the observed X-ray column density in some GRBs, although the contribution from the host galaxy alone cannot be excluded based on this study.
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